[ Upstream commit 190f056fba230abee80712eb810939ef9a8c462f ]

While "s390/vdso: avoid 64-bit vdso mapping for compat tasks" fixed
64-bit vdso mapping for compat tasks under gdb it introduced another
problem. "compat_mm" flag is not inherited during fork and when
31-bit process forks a child (but does not perform exec) it ends up
with 64-bit vdso. To address that, init_new_context (which is called
during fork and exec) now initialize compat_mm based on thread TIF_31BIT
flag. Later compat_mm is adjusted in arch_setup_additional_pages, which
is called during exec.

Fixes: d1befa65823e ("s390/vdso: avoid 64-bit vdso mapping for compat tasks")
Reported-by: NStefan Liebler <stli@linux.ibm.com>
Reviewed-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Cc: <stable@vger.kernel.org> # v4.20+
Signed-off-by: NVasily Gorbik <gor@linux.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: NSasha Levin <sashal@kernel.org>